1. Field of the Invention
The present invention relates to a radio communication apparatus and a radio communication method.
2. Description of the Related Art
A radio communication system using a FDMA (frequency division multiple access) method or the like has been conventionally known. In particular, a radio communication system using an OFDMA (orthogonal frequency division multiple access) method or the like has recently attracted attention.
In a case where there are three adjacent cells 1 to 3 in a radio communication system using the orthogonal frequency division multiple access method, the following two configurations have been known, for example. In the first configuration, the same frequency band (f MHz) is used in each of the cells 1 to 3 (see FIG. 1). On the other hand, in the second configuration, a certain frequency band (f MHz) is divided into three frequency bands (f/3 MHz), and then the divided frequency bands (f/3 MHz) are allocated respectively to the cells 1 to 3 (see FIG. 2).
In the first configuration (see FIG. 1), the whole of a certain frequency band is used by a plurality of cells (here, the cells 1 to 3). Accordingly, when interference from other cells is small, a high peak throughput can be achieved. However, since the same frequency band is used also in the adjacent cells, the inter-cell interference becomes large. As a result, a sufficient communication quality (transmission rate, call loss probability, etc) cannot be provided to a mobile terminal (user) located at the edge of the cell.
On the other hand, in the second configuration (see FIG. 2), different frequency bands are used respectively in adjacent cells (here, the cells 1 to 3). Accordingly, inter-cell interference can be easily suppressed. However, since a certain frequency band is divided into three frequency bands, the maximum peak throughput is one third of the peak throughput of a cell using the whole of the frequency band. As a result, there is a problem that radio resources cannot be sufficiently utilized when traffic is not uniform among the cells.
Consequently, the following configuration has been proposed (see Japanese Patent Application Publication No. 2005-80286). In this configuration, cells are each divided into an outer region and an inner region as shown in FIG. 3. Then, a frequency band F4 that is commonly used in plural cells is allocated to the inner regions where interference from other cells is small. On the other hand, frequency bands F1 to F3 that are separately used in the respective cells are allocated to the corresponding outer regions where interference from other cells is large.
The above-described conventional techniques are aimed at audio communications in which time variation in inter-cell interference is relatively small.
By contrast, data communications have the characteristic that the variation in interference to the surrounding environment is large due to the feature of traffic of the data communications in which short packets are intermittently transmitted, particularly in a point-to-point link.
On the other hand, the data communications also has the characteristic that retransmission processing can be performed. However, since the transmission power and MCS (modulation coding sets) are determined based on the interference level estimated prior to transmitting the data, there is a problem that a condition with a large variation in the amount of interference is not favorable.
Moreover, in a case where the OFDMA method is supposed to be used, it is possible to implement both of a “perfectly-orthogonal channel (described later)” and a “quasi-orthogonal channel (also described later)” within the same frequency band of the same radio communication system, while it is impossible in the case of the FDMA method.